This invention relates to liquid-filled vibration isolators for damping vibrations by flow resistance of a liquid enclosed therein and particularly relates to structures of liquid chambers and orifice channels.
An example of well-known vibration isolators of such kind is an engine mount for a motor vehicle. The engine mount has a basic structure in which a first fitting located on the engine side (connected to a supported part) is connected to a second fitting located on the vehicle body side (connected to a supporting part) by a rubber elastic material, a plurality of liquid chambers are formed to change their volumes with deformation of the rubber elastic material and the liquid chambers are communicated with each other by an orifice channel. By using resonance of the liquid flowing via the orifice channel between the liquid chambers, engine vibrations in a predetermined frequency range can be effectively absorbed and/or damped.
Generally, because of use of vehicle engines over wide operating range, engine mounts are desired to provide a vibration isolation effect on input vibrations of different frequencies and amplitudes. However, the frequency of vibration effectively absorbed or damped by liquid flow via the orifice channel as described above is almost determined by the cross-sectional area or length of the orifice channel. Therefore, a sufficient vibration isolation effect on few kinds of input vibrations cannot be obtained by a single orifice channel.
To cope with this, for example, Patent Documents 1 and 2 disclose techniques in which two orifice channels of different cross-sectional areas or different lengths are provided to tune them to different frequency ranges. Specifically, a vibration isolator disclosed in Patent Document 1 includes: a main liquid chamber; a first sub liquid chamber separated from the main liquid chamber by a partition member; and a second sub liquid chamber formed on the main liquid chamber side of the partition member. Furthermore, a first orifice channel providing communication between the main liquid chamber and the first sub liquid chamber is tuned, for example, to engine shake at a frequency below 15 Hz, while a second orifice channel providing communication between the first and second sub liquid chambers is tuned, for example, to engine-idle vibration at a frequency of 20 to 40 Hz.
In addition, when the vibration isolator inputs vibration at a frequency over 40 Hz, an elastic film member (membrane) separating the main liquid chamber from the second sub liquid chamber deforms to absorb variations in the liquid pressure of the main liquid chamber. Thus, booming noise in the passenger compartment can be reduced.
In a liquid-filled vibration isolator disclosed in Patent Document 2, unlike that in Patent Document 1, the second sub liquid chamber is formed on the first sub liquid chamber side of a partition member separating the main liquid chamber from the first sub liquid chamber. A second diaphragm made of rubber separates the second sub liquid chamber from the first sub liquid chamber. Furthermore, a first orifice channel providing communication between the main liquid chamber and the first sub liquid chamber is tuned to engine shake at a frequency of approximately 10 Hz, while a second orifice channel providing communication between the main liquid chamber and the second sub liquid chamber is tuned to engine-idle vibration at a frequency of approximately 20 to 30 Hz.    Patent Document 1: Japanese Patent No. 3461913    Patent Document 2: Japanese Patent No. 3563309